What are Primary Keys, Unique Keys and Foreign Keys?

Primary key plays a very important role in a table. A Primary Key is the one which uniquely identifies a row of a table. A primary key is used to create a relationship between two tables. This key does not allow duplicate values and also does not allow null values.

For example:

     UserId   FirstName      LastName
        1      John               Smith
2      Simon             Douglas
3      Bob                Franklin

It will not the values as follows:

     UserId    FirstName      LastName
        1       John               Smith
1       Simon             Douglas
2       Bob                Franklin 

Unique key is similar to Primary Key. Unique key is the one which uniquely identifies a row of a table. It will not allow duplicate values but it allows null values. So, it’s advisable to declare a primary key.

A foreign key is a field that points to the primary key of another table.

For example:

PersonalDetails_table                     RegistrationDetails_table

UserId                                         ID
FirstName                                           UserId
LastName                                           RegistrationDate

In the above relation, UserId field is common in both the tables. The field UserId is a primary key of PersonalDetails _table and a foreign key of RegistrationDetails_table.